1. Field of Invention
This invention relates to the field of mass spectrometry. More specifically, this invention relates to procedures and devices for transporting of ions created at/or near atmospheric conditions into vacuum of a mass spectrometer.
2. Discussion of the Background
Multiple ionization techniques used in modern mass spectrometry operate at atmospheric pressure. To achieve the maximum sensitivity, ions must be transmitted with high efficiency through differentially pumped vacuum chambers into high vacuum region of the mass analyzer. The most challenging of the steps involved is ion transmission in the first chamber, which typically operates in Torr pressure region.
Due to demand for highly sensitive mass spectrometers with atmospheric pressure interfaces (API), there is a great interest in developing ion guide systems for transfer of ions at elevated pressures (≧1 Torr). Operation of interfaces at elevated pressures will permit the use of vacuum pumps with lower pumping speed to obtain the same gas intake or alternatively to increase the gas load through API using pumps with the same pumping speed.
There are two types of ion guides in which an alternating (radio frequency, or RF) electric field are used for trapping (focusing) ions in radial (transverse) direction along the ion pathway. The segmented ring electrode ion guide and its variations like ion tunnel or ion funnel (where the orifices in the ring electrodes vary along the ion pass way) are examples of the first type. Multipole ion guides having rod electrodes located along the ion pathway represent the second type. A direct current (DC) electric field for pushing (transferring) ions along the ion pathway can be created in both ion guide types.
Currently, ion funnels are used for transferring (and focusing) ions at elevated pressures. These devices can be made of very thin (e.g., less than 0.5-1.0 mm) metal rings separated by insulators of comparable thickness. This small step in ring electrode position along with high frequency (e.g., as high as 1.74 MHz) of the trapping RF voltages applied to the ring electrodes accounts for high efficiency of trapping ions in radial direction by ion funnel devices at elevated pressures as high as 29 Torr. See Smith et al. in the J Am Soc Mass Spectrom 2006, 17, 1299-1305, the entire contents of which are incorporated herein by reference.
Prior to this invention, multipole ion guides were not utilized for operations at elevated pressures >5-10 Torr, probably because the focusing properties of ion guides deteriorates at these higher pressure due to increasing number of defocusing gas collisions and due to an operational limit caused by gas discharge formation at higher RF voltages. Prior to this invention, the maximum operational pressures of multipole ion guides were typically in 1-2 Torr range. See Collins et al. U.S. Pat. No. 7,259,371, the entire contents of which are incorporated herein by reference. The mean free path λ is about 0.2 mm at these pressures, which is comparable to a typical conductance limit diameter. At pressures of 1-2 Torr, gas flow in a region of the ion guide is far from a continuum gas flow regime and is close to a free molecular regime.